(Not Applicable)
1. Field of the Invention
This invention relates to the measurement of particulate matter. More specifically, this invention relates to the in-situ, real-time measurement of elements associated with aerosols.
2. Description of the Relevant Art
Several metals, such as mercury, arsenic, and chromium, among others, are toxic to human health and to certain ecosystems. The United States Environmental Protection Agency has planned to regulate the emission of these, as well as other, metals. Proper determination of compliance with these potential regulations requires measurement of metal emissions to be made. Traditional means for detecting or measuring these and other emissions are time-consuming, labor intensive, and extremely costly. Traditional detecting or measuring means used filter-based time-integrated sampling schemes followed by laboratory analysis by trained laboratory technicians. These measurements typically take a period of several hours to perform, and offer little use to continuous emission monitoring, pollution control, and/or particle toxicological research that requires a large number of data measurements to be made on a high volume of emissions very often. Additionally, a variety of sampling artifacts could exist that complicate collection of representative data. For instance, in-situ nucleation and coagulation can alter particle size distribution and chemical composition as a function of particle sizes, leading to inaccurate data when dilution sampling is employed. Solving such a complication is difficult, and no general solution is available. These schemes are also problematic in that they are not particularly good for use in hazardous environments, such as those that are hot, radioactive, and/or oxygen deficient. This forecloses the adequate measurement of elements in environments in which the monitoring of particulate matter is particularly important.
Real-time, in-situ measurement is the best and probably the only solution for detecting or measuring particulate matter in a hazardous environment. Real time physical and chemical classification of particulate matter can provide direct measurements of the dynamics and phase partition, as well as the transformation of aerosols and chemical species associated with aerosol particles. However, there is currently no commercially available instrument for the continuous measurement of emissions in a hazardous environment, among others.
A compact laser-based instrument was developed for detecting or measuring elements found in aerosol particles. This instrument made measurements of these elements using laser-induced plasma spectroscopy (LIPS). LIPS is an established technique for detection of metals in various matrices such as solid, liquid, gas, and/or aerosol particulate matter. LIPS requires no alteration of the condition of a sample, because the measurement of samples is performed in situ. Furthermore, LIPS has a response time on the order of seconds, which is extremely short and which allows the LIPS system to perform measurements in real time. Applications of LIPS systems have been limited, however, due to their lack of sensitivity as compared to other atomic emission spectrometric techniques, such as inductively coupled plasma/atomic emission spectroscopy (ICP/AES), among others.
The LIPS technique has several advantages over the other traditional analytical techniques for particle measurement. These advantages include rapid turn-around time, non-invasive, in-situ, flexible configuration, and the readiness of a LIPS device for use in building a compact trace metal analyzer. These strengths make LIPS an attractive candidate for the development of a field-portable, multi-element monitor for use in a hazardous environment such as a radiological hot cell, a mix-waste contaminated area, or a high-temperature combustion chamber. A LIPS with a high degree of sensitivity may also be a good instrument for performing environmental and/or health research. Unfortunately, a LIPS for these uses is not commercially available at the current time.
A problem with the LIPS technique is that it cannot, on its own, detect metals, such as mercury or chromium, among others, in aerosols at a level commonly found in source emissions. The LIPS technique, on its own, has never before detected mercury or chromium in a field test. To raise the instrument""s analytical performance, a number of proposals were tested. In Sattman, R. et al. (1995) J. Phys. D.:Appl. Phys., 28, 2181-2187, the use of double or multiple laser pulses to achieve higher signal-to-noise ratios for detecting Si in solid steel samples was demonstrated. It was found that double pulses enhanced the signal over single pulses used in traditional LIPS by 2 orders of magnitude. In Gornushkin et al. (1997), Appl. Spectrosc., 51(7):1055-59, the use of a LIPS/Laser-excited atomic fluorescence spectrometry technique for the determination of cobalt in solid sample matrices, such as graphite, soil, and steel, was suggested. They found the combination offered a technique that has linearity over four orders of magnitude in the ppb to ppm range, and the analytical result was comparable to ICP/AES. However, while these sensitive techniques provide greater detection levels for metals and appear to yield better analytical results than a single-pulse LIPS, they still do not provide a way to make a LIPS-based device that can be miniaturized, user friendly, and field portable. Additionally, these techniques have not been demonstrated to be applicable to aerosol measurement.
It is an object of the invention to provide a method and apparatus for the continuous in situ detection or measurement of elements associated with particulate matter.
It is another object of the invention to provide a method and apparatus for the real-time detection or measurement of elements associated with particulate matter.
It is still another object of the invention to provide an apparatus for the detection or measurement of elements associated with particulate matter that is reduced in size and preferably portable.
Another object of the invention is to provide a method and apparatus for detecting or measuring elements associated with particulate matter that has acceptable precision.
These and other objects of the invention are achieved by the subject apparatus for detecting elements in a given environment, which comprises an aerosol beam focuser for concentrating an aerosol into an aerosol beam and a laser and a detection device for detecting the elements found in the aerosol beam and measuring the quantities of these elements. The laser induces the initiation and formation of plasma from the aerosol beam. The detection device may comprise, among other things, a spectrometer for detecting spectral emissions of the aerosol particles caused by the initiation and formation of the aerosol particles into plasma. The apparatus may further comprise a processor for correlating the spectral emissions caused by the formation of plasma with the elements and the amounts of the elements corresponding to these spectral emissions. This apparatus may also comprise an aerosol generator for producing aerosol particles from bulk particulate matter.
In the present invention, the aerosol beam focuser is preferably a nozzle. In the present invention, the detection device is preferably a laser-induced plasma (or breakdown) spectrometer (LIPS). The laser used in the LIPS is preferably a high-energy laser beam. Furthermore, the beam of the laser used in the LIPS preferably has a short pulse. The spectrometer used in the LIPS is preferably a combination of at least one grating and an intensified charge-coupled device.
An additional embodiment of the invention is a method for detecting elements in a given environment, which comprises concentrating an aerosol into an aerosol beam, and detecting the elements and measuring the amounts of the elements in the aerosol, among other things. The step of detecting the elements and measuring the amounts of the elements in the aerosol can also further comprise the steps of forming plasma from the aerosol, and detecting spectral emissions caused by the formation of plasma to determine the identity of the elements contained in the aerosol. The step of detecting the elements and measuring the amounts of the elements in the aerosol beam can further comprise the step of measuring the intensity of the spectral emissions caused by the formation of plasma to determine the amount of the elements contained in the aerosol beam.
There is no specific restriction as to the manner in which plasma can be formed from the aerosol particles. In the present invention, the aerosol particles are preferably transformed into plasma through the use of a laser. The laser preferably has a short pulse and emits a high-energy beam. Furthermore, the aerosol particles are concentrated and the laser beam is preferably fired and focused through an optical lens array, among other things, so that the aerosol particles and the laser beam intersect one another. Such an intersection allows the use of a comparatively lower energy laser for transforming the concentrated aerosol particles found in the aerosol beam into plasma, than had the aerosol particles not been concentrated.
There is no specific restriction as to the manner in which the identity and amount of the elements contained in the plasma can be measured. When the laser transforms the aerosol beam into plasma, light is emitted which can be detected and measured to determine the identity and amount of the elements present in the plasma. Consequently, the identity and amount of the elements present in an aerosol beam can be determined. In the present invention, such detection and measurement is preferably made using spectroscopy.